1. Field of the Invention
The present invention relates to a fixing device for use in an electrophotographic image forming apparatus such as a copier, a printer, or a facsimile and, more particularly, to a fixing device for fixing a toner image to a recording medium by utilizing low-frequency induction heating.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a copier, a printer, or a facsimile is provided with a fixing device for fixing a toner image formed on a sheet as a recording medium to the sheet.
Although various systems have been used to implement the fixing device, there has been proposed a fixing device in an induction heating system to satisfy the recent request for energy conservation. The system is more efficient than a fixing system using a halogen lamp as a heat source which has been currently in widespread use.
As disclosed in, e.g., Japanese Unexamined Patent Publication No. Hei 10-207265, a fixing device in an induction heating system comprises: a hollow conductive member; a iron core partly inserted through the hollow conductive member to form a closed magnetic circuit; and an induction coil wound around the iron core. By allowing an alternating current to flow through the induction coil, an induction current is generated in the hollow conductive member, thereby inductively heating the hollow conductive member.
Such a fixing device in an induction heating system is internally provided with a iron core which forms a closed magnetic circuit. The mounting of the iron core requires an installation capacity and causes the problem of a larger-sized device.
In addition, the iron core is heavier in weight than a halogen lamp or the like. Therefore, a member for holding the iron core should have sufficient rigidity to withstand the heavy weight, which leads to higher cost.
In a structure in which the conductive member is formed on a roller, the roller has a large diameter to conform to the cross-sectional area of the iron core. This increases a curvature on a surface of the roller so that the sheet after fixation is less likely to be separated from the surface of the roller.
In view of the foregoing circumstances, it is therefore an object of the present invention to provide an improved induction heating fixing device.
Another object of the present invention is to reduce the size and weight of the induction heating fixing device by providing a smaller-sized closed magnetic circuit iron core.
Still another object of the present invention is to provide an induction heating fixing apparatus which allows smooth discharge of a sheet after fixation and is less likely to suffer a sheet jam.
To attain the above and other objects, an induction heating fixing device in accordance with an aspect of the present invention comprises: a conductive member; a core forming a closed magnetic circuit; and an induction coil provided around the core to generate an induction current in the conductive member. The conductive member, core, and induction coil are formed in stacked thin layers. This reduces the size and weight of the entire fixing device. Since the weight of the fixing device itself is reduced, it is no more necessary to use such a member with high rigidity as used in the conventional induction heating fixing device, which offers a cost advantage.
The conductive member, core, and induction coil stacked in layers are formed as an endless flexible belt. As a consequence, the sheet after fixation can be separated from a surface of the belt more successively than in a fixing device in a roller system.